1. Field of the Invention
The present invention relates to a method and an apparatus for controlling power in a wireless communication system. More particularly, the present invention relates to a method and an apparatus for controlling power according to a terminal's communication environment in a wireless communication system.
2. Description of the Related Art
Wireless communication system specifications provide references regarding an Error Vector Magnitude (EVM) of transmission signals according to a modulation order. As used herein, the EVM is an index of a required quality for transmission signals, and indicates a difference between a reference waveform and a measured waveform in connection with an output signal from a radio frequency amplifier.
In a wireless communication system, generally, a decrease in the EVM corresponds to an increase in quality of the transmission signal, but the decrease in the EVM increases a cost of manufacturing and a complexity of implementing an amplifier in a base station of a wireless communication system. Therefore, the implementation is generally made at such a level that satisfies the EVM required by specifications. For example, 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) specifications require that the EVM be equal to or less than 17.5% for Quadrature Phase Shift Keying (QPSK), 12.5% for 16 Quadrature Amplitude Modulation (QAM), and 8% for 64 QAM.
The EVM needs to satisfy required conditions when the base station is operated at a maximum output. When the amplifier is operated at low output, the amplifier's linearity improves, and the EVM decreases. However, the low output reduces the base station's coverage and degrades the downlink signal quality.